Promoting a freeware game will be different from promoting a commercial game – the most important thing, I think, is to be aware of who your target audience is and to make sure they see your message.
Forum signatures are a great place to advertise since it is essentially free space, and if you are an active poster it will quickly spread your message across many different topics.
You can obviously start a topic promoting your game in the GameMaker creations section of the GMC, and you could also post your website in the Website Announcements section to coincide with your release. There isn’t just the GMC though, if you are a member of any other forums which focus on GameMaker or game development you should also try the same there.
Affiliating with other sites is a good idea but you only get a small canvas to show your message on using this method. If your banner will appear at the bottom of a page alongside hundreds of others this will also be pretty useless (says the man who runs a Game Maker affiliation service).
I believe it would be more beneficial to try to get your game reviewed or previewed by one or, even better, both of the GameMaker magazines – provided you have put time into your game your review should in effect be free advertising and you will also get some constructive criticism on top of that. These magazines also give free advertising in exchange for articles so if you get writing about something GameMaker related, and not just shameless promotion, and submit it to one of them you will probably be given a quarter page ad or something similar. From my experience ads in MarkUp do get clicked and do get results.
As well as choosing the right place to advertise your message also has to be effective, you will only have a limited amount of space so you need to make the most of it and make sure you have got everything right.
Modified from a post I made at the GMC.